Field of the Invention
The present invention relates to an electric machine having a superconductive rotor provided with a cooling circuit having a seal.
In such machines, the cooling problem is particularly great because of the high cooling powers made necessary by the use of very low temperatures.
Such a machine is, for example, a cryoalternator. Its rotor, which acts as an inductor includes superconductive windings at its periphery. These windings must be kept at a very low temperature despite the heat which is generated both by electric losses in the windings and by radiation or thermal conduction.
For this reason, it has been proposed to provide the rotor with an inner cooling circuit supplied with helium at a temperature close to 4.degree. K. The helium can be supplied by an axial rotating supply tube integral with the rotor and extended by a stationary supply tube which is also axial by means of a rotating seal. The helium can be recuperated at the outlet, cooled by a cooling device and re-injected into the supply tube by circulation means, i.e. a pump capable of operating in the special conditions required.
However, the man in the art will find numerous difficulties when he wants to produce such a cooling circuit. He can, indeed, contrive three distinct general arrangements. In a first arrangement the helium is recuperated at ambient temperature. It is then difficult, in the majority of cases, to obtain acceptable thermodynamic efficiency in the cooling system. Furthermore, systems of this type produced or described up to now include accurate guide means which are hardly compatible with the operation of an industrial machine whose availability is an essential factor. In this respect, the following documents may be quoted.
The thesis by W. DAVID LEE submitted in June, 1970, to the Massachusetts Institute of Technology (Us) entitled "Continuous transfer of liquid helium to rotating dewar" shows in a simplified theoretical approach that with laminar flow of helium in the annular gap of a bayonet coupling, the thermal losses by gaseous convection are proportional to the ninth power of the radial gap between the two parts of the seal and to the cube of the inner radius of the annular space.
The publication "Applications of superconductivity to AC rotating machines" by L. Smith et al., NATO conference 1973, superconducting Machines and Devices, NATO Advanced Study Series, Series B: Physics, vol. I, Plenum Press N. Y. which describes a cryogenic seal whose accurate guiding by means of ball bearings limits the radial play of the bayonet to 0.3 mm in order to avoid any cold sealing packing.
The publication of S. Akiyame et al. "The model rotor for the 6 MVA superconducting generator", L. 6 Sixth International Cryogenic Engineering Conference (I.C.E.C.6) May, 11-14, 1976, exists the rapport of which will be published by I.P.C. Service and Technology Press Ltd., 32, high Street, Guildford, Surrey, England.
All these documents seem to infer that the play of a rotating seal has a very great influence on the efficiency of the cooling system and that a minimal play is necessary to avoid cold floating linings.
If it is required to recuperate the helium at a temperature close to its input temperature, a second rotating seal must be used at the output. According to a second general arrangement, this second rotating seal could be placed on the opposite end of the rotor to that of the first rotating seal. This is then a hindrance in the case of a powerful machine on account of the presence of the drive shaft transmitting the mechanical power of the rotor. According to a third general arrangement, the second rotating seal can be placed at the same end of the rotor as the first. It is then necessary to use either a complex arrangement of seals, or to dispose one of the seals internally, i.e. to put it inside the other or inside pipes leading to the other and therefore immerse both its inner and its outer face in helium at a temperature close to 4.degree. K. Taking into account its temperature, the rotation speeds used and the materials available, it is difficult to seal it adequately. The outer rotating joint, however can be sufficiently sealed by means of friction packings and of conventional guide bearings which operate at a temperature close to ambient temperature, since the outer seal surrounds the inner seal and can therefore have its outer face in thermal contact with the atmosphere. It results from this contact however, that the outer seal must have as small a diameter as possible to avoid thermal losses.
As for the inner rotating seal, it must allow a correct circulation, in the rotating rotor, of the helium which has passed through the stationary outer cooling device, this being applicable at the various rotation speeds of the rotor, e.g. 50 r. p. m. It is known, in an industrial sized machine, that there is a lashing action and shaft jumping action which are extremely difficult to suppress and which are incompatible with the forming of a more or less fluid-tight rotating seal. Indeed, the materials used at these temperatures cannot withstand the friction of the rotating part of the seal on its stationary part, even discontinuously. In practice, in the case of a bayonet seal, a radial play of more than 0.3 mm has never been considered. Now, in the case of a high-power machine, a lashing amplitude and shaft jump which are smaller than this radial play could only be obtained by means of extraordinary manufacturing precision which would certainly be very expensive, or by using guide bearings which are very close to the rotating seal and are very cumbersome there. In the case of a superconductive rotor cooled by means of a circuit of a known type, the defective sealing of an inner rotating seal would be revealed by the fact that the majority of the helium set in motion by the circulation means looped back on itself across the leakage of the inner seal, without entering the rotor. The result of this would be very poor efficiency of the cooling device, whose operation already expends a great amount of energy because of the very low temperatures used. That is why, despite numerous studies made throughout the world for producing a cryoalternator, none of the three general arrangements which come to the mind of the man in the art and which have been set forth hereinabove have been developed before the present invention into a device which could be used in practice for cooling a rotor of a high-power cryoalternator (above a few hundred megawatts) with sufficient reliability for the operating of this type of machine.
Preferred embodiments of the present invention produce an electric machine having a superconductive rotor provided with a cooling circuit fitted with rotating seals which make it possible to obtain effective cooling of the rotor simply.